As conventionally known, in an iron and steel manufacturing process, iron-containing pickling waste liquid is discharged from a bath of a pickling device in a steel pickling line, and the pickling waste liquid is roasted in a roasting furnace to produce iron oxide (Fe2O3) and acidic gas such as hydrochloric acid. The produced iron oxide is recovered as red iron oxide, raw materials for soft ferrite and hard ferrite, or the like. On the other hand, the furnace gas containing hydrochloric acid or the like is brought into contact with water to be subjected to an acid recovery process, and the recovered acid is returned to the bath of the pickling device (Non-Patent Literature 1).
In conventional iron and steel manufacturing processes, various processing methods and devices have been proposed or improved only from the perspective of efficient processing of produced waste liquid, including recovery of acid at low cost or high yield, recovery of iron oxide at high purity and/or high yield, and the like, for example (Patent Literatures 1 to 3).
Moreover, consideration has been given to manufacturing multiple oxides for soft ferrite by adding chloride of Mn, Mg, Ni, Zn, or the like, which are constituent metal elements of soft ferrite composition, to the iron and steel pickling waste liquid and then roasting the mixture (Patent Literatures 4 to 6).
As described above, the intended products in the course of conventional process to the iron and steel pickling waste liquid are acid, and iron oxide or soft ferrite, and there are very few examples of attempts to manufacture products other than the above materials from the pickling waste liquid.